JP6358085B2 - Method for identifying magnetic performance of rare earth magnets - Google Patents

Description

  The present invention relates to a method for specifying the magnetic performance of a rare earth magnet.

  Rare earth magnets using rare earth elements such as lanthanoids are also called permanent magnets, and their uses are used in motors for driving hard disks and MRI, as well as drive motors for hybrid vehicles and electric vehicles.

  Residual magnetization (residual magnetic flux density) and coercive force can be cited as indicators of the magnet performance of this rare earth magnet. However, in response to increased heat generation due to miniaturization of motors and higher current density, rare earth magnets used also The demand for heat resistance is further increasing, and how to maintain the magnetic properties of the magnet under high temperature use is one of the important research subjects in the technical field.

  As rare earth magnets, in addition to general sintered magnets with a crystal grain (main phase) scale of 3 to 5 μm constituting the structure, nanocrystal magnets with crystal grains refined to a nanoscale of about 50 nm to 300 nm are available. Among them, nanocrystal magnets that can reduce the amount of expensive heavy rare earth elements added or eliminate the addition of heavy rare earth elements while miniaturizing the crystal grains described above are currently attracting attention.

  An example of a method for producing rare earth magnets is outlined below. For example, a magnetic ribbon produced by pulverizing a quenched ribbon (quenched ribbon) obtained by rapidly solidifying an Nd-Fe-B metal melt. In general, a method of producing a rare earth magnet (orientated magnet) by forming a sintered body while being hot-pressed into a sintered body and subjecting the sintered body to hot plastic working to give magnetic anisotropy is generally applied. ing.

  By the way, it is already known that a magnet such as a rare earth magnet has a plurality of structural parameters that contribute to improving its performance. For example, in order to obtain a high residual magnetic flux density, a higher degree of crystal orientation is better. In addition, in Nd—Fe—B series magnets, it is preferable to have a higher volume ratio, and the like is given as an example of the knowledge, and these are also explained theoretically. However, these findings mainly start from the explanation about the influence of a single parameter, and there has never been any knowledge that comprehensively determines the contribution of each parameter to the magnetic performance of the rare earth magnet.

  Here, Patent Document 1 discloses a magnetic property calculation method that calculates the coercive force distribution in the thickness direction of the magnet from the measurement value of the BH tracer, and calculates the magnet without division, without actually dividing the magnet. Yes.

  According to this magnetic force characteristic calculation method, the coercive force distribution can be easily calculated in a short time without destroying the magnet. Although it can be said that this calculation method measures and evaluates magnetic characteristics macroscopically, the influence of the microscopic structure is not taken into consideration at all. Therefore, it merely remains to specify macroscopic magnetic characteristics, and there is no way of knowing which tissue factor can be used to obtain a high-performance magnet. Therefore, there is naturally no disclosure of a technique for evaluating that a plurality of tissue factors influence each other to determine the magnetic characteristics of the entire magnet.

JP 2013-36904 A

  The present invention has been made in view of the above-described problems, and a rare earth magnet capable of accurately identifying the magnetic characteristics of a rare earth magnet by comprehensively determining the influence of a plurality of microstructures constituting the magnet. An object of the present invention is to provide a method for specifying magnetic characteristics.

In order to achieve the above object, the magnetic performance of the rare earth magnet according to the present invention is determined by pressing a magnetic powder for a rare earth magnet to produce a sintered body, which has a magnetic anisotropy. A method for identifying the magnetic performance of a rare earth magnet to identify the magnetic performance of a rare earth magnet manufactured by applying hot plastic working, wherein A: equiaxed grain structure, B: shear orientation structure, C: Oxide / nitride structure, D: Flat grain orientation structure, remnant magnetization: Br is specified by the following formula: Br = Br (A) + Br (B) + Br (C) + Br (D), where Br (A) is the volume% of structure A × orientation degree of structure A × main phase ratio of structure A × theoretical highest Br value of rare earth magnet , and Br (B) is structure B. Volume% × orientation degree of structure B × main phase ratio of structure B × theoretical maximum Br value of rare earth magnet , Br (C) is volume% of structure C × 0, Br (D) is the structure D Volume% x degree of orientation of structure D x main phase ratio of structure D x rare earth magnet It is a method for specifying magnetic performance, defined by the theoretical maximum Br value of stone .

  The method for specifying the magnetic properties of the present invention categorizes crystal structures into four types, calculates the residual magnetization Br of each structure, that is, the product of volume and orientation, and obtains the sum to obtain the magnetic properties of the entire rare earth magnet ( Among them, the residual magnetic flux density and residual magnetization are specified.

  The A: equiaxed grain structure in the crystal structure is a crystal structure having an aspect ratio of less than 4 when approximated to an inertial ellipse, for example. When the aspect ratio is small (for example, less than 4), the degree of orientation is low, and when the aspect ratio is large (for example, 4 or more), the degree of orientation is high.

  On the other hand, the B: shear orientation structure in the crystal structure is a crystal structure in which the orientation around the crack that is approximately perpendicular to the C-axis direction of the oriented crystal group is disordered, for example, a structure that looks like a diamond shape. .

  On the other hand, the C: oxide / nitride structure in the crystal structure is a crystal structure of Nd oxide, Nd nitride, or the like precipitated at the crystal grain boundary or the powder grain boundary.

  Furthermore, the D: flat grain oriented structure in the crystal structure is a crystal structure having an aspect ratio of 4 or more when approximated to an inertia ellipse.

Particular method of the magnetic properties of the present invention, the four types of calculating a product of the crystal volume% and orientation degree of the product of tissue, and the main phase ratio and the theoretical maximum Br value of the rare earth magnet of the crystal structure, both the product Is calculated as the residual magnetic flux density of each structure, and the sum of the residual magnetic flux densities of the four types of crystal structures is calculated as the residual magnetic flux density of the entire rare earth magnet.

  Here, in the case of a Nd—Fe—B rare earth magnet, the theoretical maximum Br value is 1.61T.

The main phase ratio is a volume fraction of, for example, Nd ₂ Fe ₁₄ B crystal, which is a hard magnetic phase in the material, and main phase ratio = (1−grain boundary phase ratio) × (1−porosity). ).

  In addition, regarding C: oxide / nitride structure, the main phase is not included because of oxide and nitride, and there is no contribution to the residual magnetic flux density. Therefore, since the contribution to the decrease in residual magnetic flux density can be estimated, the calculation has a great meaning.

  As can be understood from the above description, according to the method for specifying the magnetic characteristics of the rare earth magnet of the present invention, the influence of the multiple types of microstructures constituting the rare earth magnet on the magnetic characteristics of the entire rare earth magnet is quantitatively calculated. By summing up the calculated values, the magnetic properties of the entire rare earth magnet, particularly the residual magnetic flux density, can be specified with high accuracy.

It is the schematic diagram explaining that the rare earth magnet was represented by four types of microstructures. It is a figure explaining the calculation flow of the amount of main phases. It is the figure which showed an example of the calculation sheet | seat applied to calculation of the amount of main phases. 4 is a SEM image photograph of four types of microstructures, (a) is a photograph of an equiaxed grain structure, (b) is a photograph of a shear orientation structure, and (c) is an oxide / nitride. It is a photograph figure of a structure, and (d) is a photograph figure of a flat grain orientation organization. (A) is the figure which showed the experimental result which compares the measured value of the residual magnetic flux density measured by VSM (vibration sample type magnetometer) with the residual magnetic flux density of the rare earth magnet specified by the specific method of this invention. (B) is the figure which specified the contribution of each crystal structure in the residual magnetic flux density of the rare earth magnet specified by the specific method of the present invention. It is the figure which showed the factor analysis result which is reducing the residual magnetic flux density.

  Embodiments of a method for specifying the magnetic performance of a rare earth magnet according to the present invention will be described below with reference to the drawings.

(Embodiment of method for specifying magnetic characteristics of rare earth magnet)
FIG. 1 is a schematic diagram illustrating that a rare earth magnet is represented by four types of microstructures. First, the manufacturing method of the rare earth magnet M shown in FIG. 1 will be outlined.

  Quenching thin ribbons, which are fine crystal grains, are produced by liquid quenching and pulverized to produce magnetic powder. Specifically, for example, in a furnace (not shown) depressurized to 50 kPa or less, a melt spinning method using a single roll melts an alloy ingot at high frequency, and a molten metal having a composition that gives a rare earth magnet is jetted onto a copper roll to quench the ribbon. (Quenching ribbon) is manufactured.

  Here, the composition of the quenched ribbon is that the main phase of the RE-Fe-B system (at least one of RE: Nd and Pr) and the RE-X alloy around the main phase (X: metal element and heavy For example, when this is a nanocrystalline structure, it is composed of a main phase having a crystal grain size of about 50 nm to 300 nm.

  The Nd—X alloy constituting the grain boundary phase is composed of Nd and at least one alloy of Co, Fe, Ga, Cu, Al, etc., for example, Nd—Co, Nd—Fe, Nd— One of Ga, Nd—Co—Fe, and Nd—Co—Fe—Ga, or a mixture of two or more of these, is in an Nd-rich state.

  The produced rapidly cooled ribbon is collected and coarsely pulverized to produce a magnetic powder. The particle size range of the coarsely pulverized magnetic powder is adjusted to be in the range of 75 to 300 μm, for example.

  Next, the magnetic powder is accommodated (filled) in a cavity of a molding die composed of a carbide die (not shown) and a carbide punch sliding in the hollow. And while pressing with a super hard punch, an electric current is sent in the pressurizing direction, and it heats by heating at about 700 degreeC (hot forming, sintering), and a sintered compact is manufactured. For example, this sintered body has a Nd-Fe-B main phase with a nanocrystalline structure (average particle size of 300 nm or less, for example, a crystal particle size of about 50 nm to 200 nm) and Nd- around the main phase. It has a structure with a grain boundary phase of X alloy (X: metal element).

  Here, the Nd-X alloy constituting the grain boundary phase of the sintered body is composed of Nd and at least one alloy of Co, Fe, Ga, etc., for example, Nd-Co, Nd-Fe, Nd One of -Ga, Nd-Co-Fe, and Nd-Co-Fe-Ga, or a mixture of two or more of these, is in an Nd-rich state.

  When the sintered body is manufactured, a rare earth magnet M having a desired degree of orientation is obtained by applying a hot plastic working while pressing with a carbide punch (not shown) to give magnetic anisotropy to the sintered body. Manufactured.

  Note that the strain rate during the hot plastic working is preferably adjusted to 0.1 / sec or more. In addition, when the degree of work (rolling rate, compressibility) by hot plastic working is large, for example, hot plastic working when the rolling rate is about 10% or more can be referred to as strong working. It is better to perform hot plastic working in the range of about%.

  The rare-earth magnet M manufactured through hot plastic working has a magnetic anisotropic crystal structure, and is oriented in the easy magnetization direction and has a residual magnetic flux density Br as shown in FIG. .

  In the method for specifying magnetic characteristics of the present invention, as shown in FIG. 1, the manufactured rare earth magnet M is classified into four types of crystal structures. Specifically, the crystal structure is classified into A: equiaxed grain structure, B: shear orientation structure, C: oxide / nitride structure, and D: flat grain orientation structure.

  Here, the equiaxed grain structure A is, for example, a crystal structure having an aspect ratio of less than 4 when approximated to an inertial ellipse.

  On the other hand, the shear orientation structure B is a crystal structure in which the orientation around the crack that is approximately perpendicular to the C-axis direction of the oriented crystal group is disordered, for example, a structure that looks like a diamond shape.

  On the other hand, the oxide / nitride structure C is a crystal structure of Nd oxide, Nd nitride, or the like precipitated at a crystal grain boundary or a powder grain boundary.

  Further, the flat grain oriented structure D is a crystal structure having an aspect ratio of 4 or more when approximated to an inertial ellipse.

  The residual magnetic flux density Br of the entire rare earth magnet M is specified by the following formula by classifying into the above four types of crystal structures.

  Br = Br (A) + Br (B) + Br (C) + Br (D) (Formula)

Here, Br (A) is the volume% of structure A x orientation degree of structure A x main phase ratio of structure A x theoretical maximum Br value of rare earth magnet , Br (B) is volume% of structure B x structure B Orientation ratio x main phase ratio of structure B x theoretical maximum Br value of rare earth magnet , Br (C) is volume% of structure C x 0, Br (D) is volume% of structure D x structure D The degree of orientation x the main phase ratio of structure D x the theoretical maximum Br value of the rare earth magnet .

  By calculating Br of each crystal structure, the influence of a plurality of types of crystal structures constituting the rare earth magnet M on the magnetic properties of the entire rare earth magnet M is quantitatively calculated, and the calculated values are added together to calculate the rare earth magnet. The magnetic characteristics of the entire M, particularly the residual magnetic flux density, can be specified with high accuracy.

(Experiment and results of verifying the evaluation results by the magnetic property specifying method of the present invention)
The present inventors manufactured Nd—Fe—B rare earth magnets, A: equiaxed grain structure, B: shear orientation structure, C: oxide / nitride structure, D: flat grain orientation structure. The residual magnetic flux density of the entire rare earth magnet is identified, and the residual magnetic flux density of the rare earth magnet is measured with a VSM (vibrating sample magnetometer). An experiment was conducted to compare the results.

  Hereinafter, the alloy composition is shown in Table 1, the quenching ribbon production conditions in Table 2, the sintered body production conditions in Table 3, the hot plastic working conditions in Table 4, and the specific method of the present invention is applied. The result list is shown in Table 5, and the magnetic characteristic result is shown in Table 6. All samples were processed into a 4 mm × 4 mm × 2 mm (easy magnetization direction) rectangular parallelepiped shape, magnetized at 8 T, and the residual magnetic flux density was measured by VSM.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

  Here, regarding the Br conversion method, it was converted as follows: Br conversion = volume / 100 × main phase ratio ((1-grain boundary phase ratio) × density / 100) × orientation / 100 × 1.61.

  In addition, a calculation method of the main phase amount based on ICP mass spectrometry and ON analysis values will be described with reference to FIGS.

From the phase diagram, the phases in the magnet are assumed to be four types: Nd ₂ Fe ₁₄ B phase, Nd phase, Nd ₂ O ₃ phase, and NdFeB ₄ phase, and each phase density, ICP mass analysis, and each analysis value by ON analysis The weight and volume fraction of each phase were calculated from the amount of each element. Here, since Cu and Al were discharged to the grain boundary phase, they were converted as Nd amounts. In addition, since Nd nitride consumes 1.5 times as much Nd as Nd ₂ O ₃ which is an oxide, N was calculated as 1.5 times the amount of O.

  As shown in FIG. 2, the main phase amount calculation method is as follows. First, input the results of analysis by ICP mass spectrometry and ON analysis in (1) of the calculation sheet of FIG. The molecular weight of each phase is calculated in 2), and then the molar ratio of each phase is calculated from the analysis result in (3) of the calculation sheet and the molecular weight of each phase, and then (4) of the calculation sheet. The amount of the main phase is calculated in (5) of the calculation sheet by converting each phase density to the mass of each phase.

  First, referring to FIG. 4, four types of microstructure SEM image photographs will be described. Here, FIG. 4 (a) is a photograph of an equiaxed grain structure, FIG. 4 (b) is a photograph of a shear orientation structure, and FIG. 4 (c) is a photograph of an oxide / nitride structure. FIG. 4 (d) is a photograph of a flat grain oriented structure.

  From the photograph of the equiaxed grain structure in FIG. 4A, it can be confirmed that the aspect ratio of the structure is relatively small. On the other hand, from the photograph of the flat grain oriented structure in FIG. It can be confirmed that the ratio is very large. Moreover, it can confirm that a crystal structure is a diamond shape from the photograph figure of the shear orientation structure | tissue of FIG.4 (b). Furthermore, from the photograph of the oxide / nitride structure in FIG. 4C, it can be confirmed that there is no main phase in this structure.

  Next, experimental results will be described with reference to FIGS. Here, FIG. 5A shows experimental results comparing the measured values of the residual magnetic flux density of the rare earth magnet specified by the specifying method of the present invention and the residual magnetic flux density measured by a VSM (vibrating sample magnetometer). FIG. 5B is a diagram in which the contribution of each crystal structure in the residual magnetic flux density of the rare earth magnet specified by the specifying method of the present invention is specified. FIG. 6 is a diagram showing the result of factor analysis for reducing the residual magnetic flux density.

  From FIG. 5 (a), when the residual magnetic flux density Br determined by the specifying method of the present invention and the residual magnetic flux density Br measured by VSM are compared, the residual magnetic flux density Br is found from the 1.2T region where the residual magnetic flux density Br is low. It can be seen that the degree of agreement over the high 1.4T region is extremely high. This proves that the specifying method of the present invention can specify the residual magnetic flux density Br with high accuracy.

  From FIG. 5 (b), the contribution of each crystal structure to the residual magnetic flux density Br specified by the specifying method of the present invention is 0.09T for the equiaxed grain structure, 0.02T for the shear orientation structure, and the flat grain orientation structure. Was 1.24T.

  In addition, with regard to the shear structure, coarse grain structure, and oxide / nitride structure, which are factors that reduce the residual magnetic flux density Br, the amount of Br decrease (compared to the case where all crystal structures are assumed) is as shown in FIG. Estimated. This estimation can identify the organization that should be addressed to meet the target residual flux density Br, but this will help reduce that option when considering measures in a variety of processes. In other words, it greatly contributes to shortening the lead time for development and cost reduction. For example, if you want to reduce the coarse grain ratio, pay attention to the amount of heat input in each process and the initial structure of the quenched powder, and if you want to reduce the volume ratio of the shear-oriented structure, the processing conditions, work, and mold during strong processing It may be possible to consider the shape.

(Consideration of microstructure state satisfying high residual magnetic flux density Br)
The inventors further manufactured rare earth magnets with the alloy compositions and processing conditions shown in Tables 1 to 4 above, and considered the structure state satisfying the high residual magnetic flux density Br.

  In general, the range of grain boundary phase required to obtain a high degree of orientation by hot plastic working, that is, the range of 0.955 ≦ Nd-Fe-B ratio ≦ 0.965, and the optimum conditions (temperature and strain It is important to perform hot plastic working under the above speed, and by producing under these conditions, the degree of orientation of the finely oriented structure ≧ 95% can be achieved.

  Ideally, the fine orientation structure is 100% in volume, and when the residual magnetic flux density Br in this case is calculated using 1.61T which is the saturation magnetization theoretical value of Nd-Fe-B, the expected value is 1.61T. X 0.955 (Nd-Fe-B ratio) x 0.95 (degree of orientation) = 1.46T. Considering the influence of 1% oxide / nitride structure, which is inevitable in the manufacturing process, 1.46T × 0.99 = 1.45T is obtained. However, in actuality, there are a large number of coarse grains and shear-oriented structures with a low degree of orientation that contribute greatly to the reduction of the residual magnetic flux density Br, and the residual magnetic flux density Br drops below 1.45T.

  This amount of decrease can be estimated from the relationship between each crystal structure obtained by the specific method of the present invention and the residual magnetic flux density Br. For the coarse grain structure, the amount of change in the residual magnetic flux density Br per 1% volume is -0.0057T. With regard to the shear-oriented structure, the Br decrease amount per 1% volume is -0.0095T. The contribution to the decrease in the residual magnetic flux density Br is larger in the case of coarse grain: shear orientation structure ≒ 1: 1.5, and from this relation, the residual magnetic flux density Br = 1.46 T-(coarse grain volume x 0.0057 + shear orientation structure) From this relationship, the magnet structure satisfying the high residual magnetic flux density Br ≧ 1.37T required for the rare earth magnet applied in the vehicle drive motor is as follows.

(Condition 1) Volume ratio of coarse-grained structure + 1.5 × (volume ratio of shear-oriented structure) ≦ 15 (volume%)
(Condition 2) Orientation degree of fine grain structure ≧ 95%
(Condition 3) Oxide / Nitride ≦ 1%
(Condition 4) Grain boundary phase amount (1-NdFeB ratio) ≦ 4.5 (volume%)

  The analysis results of each crystal structure are shown in Table 6. From the table, it is verified whether or not the above conditions are satisfied.

  Volume ratio of coarse-grained structure + 1.5 × (volume ratio of shear-oriented structure) = 9.26 (volume%) ≦ 15 (volume%).

  Further, the orientation degree of the fine grain structure = 95% ≧ 95%, which satisfies the condition 2.

  Further, the oxide / nitride structure = 0.5% ≦ 1%, which satisfies the condition 3.

  Furthermore, the condition 4 is satisfied from the amount of grain boundary phase (1-NdFeB ratio) = 3.8% ≦ 4.5 (volume%).

  The magnetic property evaluation result by VSM is Br = 1.410T ≧ 1.37T. Therefore, it can be seen that a high residual magnetic flux density Br can be obtained when each crystal structure satisfies the above conditions.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

  M: rare earth magnet, A: equiaxed grain structure, B: shear orientation structure, C: oxide / nitride structure, D: flat grain orientation structure

Claims (1)

Magnetic powder for rare earth magnets is pressure-molded to produce a sintered body, and the magnetic performance of the rare earth magnet produced by hot plastic working to give magnetic anisotropy to the sintered body is specified. A method for identifying the magnetic performance of a rare earth magnet,
The crystal structure is classified into A: equiaxed grain structure, B: shear orientation structure, C: oxide / nitride structure, D: flat grain orientation structure, and the remnant magnetization: Br is specified by the following formula. A method for determining the magnetic performance of a magnet.
Br = Br (A) + Br (B) + Br (C) + Br (D)
Here, Br (A) is the volume% of structure A x orientation degree of structure A x main phase ratio of structure A x theoretical maximum Br value of rare earth magnet , Br (B) is volume% of structure B x structure B Orientation ratio x main phase ratio of structure B x theoretical maximum Br value of rare earth magnet , Br (C) is volume% of structure C x 0, Br (D) is volume% of structure D x structure D theoretical maximum Br Nedea main phase ratio × rare earth magnet degree of orientation × tissue D Ri, Br, Br (a), Br (B), Br (C), and the unit of Br (D) is Ru T der .

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